1. Field of the Invention
The present invention relates to surface acoustic wave machines.
2. Discussion of the Related Art
Surface acoustic wave motors have been described, for example, in U.S. Pat. No. 4,562,374 of T. Sashida. The operation principle of such a motor is illustrated in the attached FIG. 1 that reproduces FIG. 1 of the Sashida patent. A progressive surface wave of the Rayleigh wave type generated at the surface of a fixed elastic body 1 causes ripples at the surface of this body. If a movable part 2 is urged against the body 1, the part 2 is driven by the displacement of apices A-A' of these ripples. Apices A-A' follow an elliptic path Q and their transversal displacement speed is associated with the oscillation frequency and with the amplitude of their displacement. In practice, the excitation frequency of the surface waves should be close to a resonance frequency of the body at the surface of which these waves are to be provided. This frequency can be modified only within a small range if it is desired to maintain a non negligible amplitude (for example approximately 10 micrometers) of the displacements of the apices perpendicularly to the surface in which these ripples are generated. A small difference in frequency with respect to the resonance frequency generates a high amplitude variation. Thus the relation between the displacement speed of apices A and A' along the direction of the arrow N and the excitation frequency is a complex non-linear relation.
The surface acoustic wave motor of U.S. Pat. No. 4,562,374 has the advantages of having a relative good efficiency, a small size and a light weight for a determined torque, and of a holding torque when stopped (i.e., in the absence of a signal, the moving part 2 is urged against the fixed part 1 thereby causing a non-negligible friction force between the two parts).
However, a drawback of this motor lies in that its speed can only be controlled by the provision of a sensor and feedback loop system. The same system is necessary when the position of the machine is to be controlled. The use of such sensors and feedback loops makes the motor and of a high cost, which inhibits the advantages of this motor.
To solve this problem, Satoshi Segawa (NEC Res. & Develop., Vol. 33, No. 1, January 1992) proposed to integrate a displacement sensor in the motor itself.
FIG. 2 of the present application reproduces FIG. 1 of the above cited article which concerns an implementation of a motor in which the stator and the rotor are constituted by annular facing parts 11 and 12 urged one against the other by a spring 13. An excitation system constituted by a piezoelectric ceramic 14 is fixed to the rear surface of stator 11 to generate surface acoustic waves therein. A lining 15 and a detecting piezoelectric ceramic ring 16 are successively disposed between the stator ring 11 and the rotor ring 12.
S. Segawa teaches that the compressions that are caused by the surface acoustic waves of the stator and are transmitted by the annular lining 15 to the detecting ring 16 allow the detection of phase shifts between the rotor and the stator, thereby indicating the angular position of the rotor with respect to the stator.
This device has the drawback of being relatively complex because it requires the use of a detector constituted by a specific piezoelectric ring whose various parts should be suitably wired together. Moreover, since this detector cannot be directly applied against the stator, an additional lining must be provided. These various additional parts increase the rotor's inertia and therefore in particular its response time. In addition, the phase detection systems require the use of complex and accurate electronic circuits and, preferably, digital systems as proposed by S. Segawa.